BG66UM failure?

My friend won three packs of BG66UM at an A-ranking tournament in July, and within the past hour, I strung using it. During the last few cross strings, the string failed. What I mean by failure is: the dropweight tensioned slowly to nearly level (ideal, where it should stop), but then stopped slowing down and immediately dropped to beyond level where the tension was LESS than the level tension. It did this repeatedly and it made the last few strings nearly impossible to string. I remember something like this from when I first started stringing (a year and a half ago?) but I haven't had anything since. Am I doing something wrong while stringing? Is the string too old? (almost a year since the prize was given) How can I prevent this?
I use a Klippermate. I was stringing a Yonex Nanospeed 9900 at 24/25. Thank you in advance for your help!